Much is known about how acoustical and accelerational stimuli reach the vertebrate inner ear, and about the 8th Nerve and CNS responses they elicit. The intervening mechanism of transduction of mechanical stimuli into electrical signals by hair cells, however, remains relatively obscure. This application proposes a study of transduction by sensory maculae from amphibian inner ears, in an in vitro system which permits visualization of living hair and supporting cells and their apical organelles. Under direct visual control, the effects of mechanical stimuli on hair bundles will be found. After determination of the stimulus input - electrical output characteristics of single hair cells by measurements of microphonics and by intracellular recording - the conductance changes associated with, and directional and ionic sensitivities of the transduction process will be examined. The intercellular junctions among hair and supporting cells will be studied by conventional and freeze-fracture electron microscopy, and electrical coupling will be sought among the various cell types. Monitoring of extracellular current flows during responses will permit inferences about the site where current enters or leaves hair cells, and autoradiography and immune labeling will be employed in an attempt to correlate this region with the site where aminoglycoside drugs block hair cell action. This experimental program would define the proximal stimulus to living hair cells, quantitate their electrical responses, examine the pathological effects of ototoxic antibiotics on cellular physiology, and locate the molecular substrate of transduction.